This invention relates generally to the fields of biology and chemistry. More particularly, the present invention relates to compositions and methods for use in control (e.g., reduction or elimination) of insect populations.
In view of the serious damage to plants and animals induced by some species thereof, there is tremendous interest in identifying compositions and methods for control of insect populations. Obvious problems with respect to the use of some organic and inorganic chemical insecticides have directed attention to the development of other approaches. One such alternative approach involves the identification of microbial insecticides which might be more species-specific than some heretofore-known chemical insecticides. Of particular interest in this regard are baculoviruses (Baculoviridae).
Two major factors pose significant barriers for the practical development of baculoviruses as microbial insecticides for use in programs aimed at numerically reducing populations of insect pests. First, most species of virus produce fatal infections in only a small group of closely related host species. This life history feature makes these insect pathogens environmentally safe for humans and other nontarget species, but it significantly limits the kinds of pests that can be controlled effectively. Second, infected hosts require a considerable amount of time to succumb, and during the interval between initial infection and death, the larvae continue to feed and to cause significant economic damage.
Autographa californica M Nucleopolyhedrosis Virus (AcMNPV) is the best studied of the baculoviruses and is known to infect the larvae of approximately 30 species in several families within the insect order, Lepidoptera Granados, R. R., and Williams, K. A. (1986). In vivo infection and replication of baculoviruses. In The biology of baculoviruses (eds. R. R. Granados and B. A. Federici) vol. I, CRC Press, Inc., Boca Raton, Fla.!. Infections are initiated when larvae ingest virion-containing proteinaceous occlusions (polyhedra) which rapidly dissolve upon contact with the highly alkaline juices within the larval midgut. Dissolution releases the enveloped occlusion-derived virus (ODV) Murphy, F. A., Fauquet, C. M., Bishop, D. H. L., Ghabrial, S. A., Jarvis, A. W., Martelli, G. P., Mayo, M. P., and Summers, M. D. (1995). Virus Taxonomy: sixth report of the international committee on taxonomy of viruses. Springer Verlag, Wien, N.Y.! which may contain as many as 29 individual nucleocapsids Adams, J. R. and Mclintock, J. T. (1991). Baculoviridae. Nuclear polyhedrosis viruses. Part 1. Nuclear polyhedrosis viruses of insects. In Atlas of invertebrate viruses (eds. J. R. Adams and J. R. Bonami), CRC Press, Inc., Boca Raton, Fla.!. Nucleocapsids enter differentiating and mature columnar cells in the midgut epithelium following fusion of the viral envelope with the microvillar membrane of these cells Kawanishi, C. Y., Summers, M. D., Stoltz, D. B., and Arnott, H. J. 1972. Entry of an insect virus in vivo by fusion of viral envelope and microvillus membrane. J. Invertebr. Pathol. 20:104-108; Granados, R. R. 1978. Early events in the infection of Heliothis zea midgut cells by a baculovirus. Virology 90:170-174!. Nucleocapsids then migrate to the nucleus where they uncoat and begin the processes of transcription and replication. Evidence suggests that some parental nucleocapsids also may be transported directly through the cell where they are re-enveloped by budding through a modified basal plasma membrane Granados, R. R., and Lawler, K. A. 1981. In vivo pathway of Autographa californica baculovirus invasion and infection. Virology 108, 297-308!.
Progeny nucleocapsids produced by infected midgut cells also bud from the basal plasma membrane containing a newly synthesized, viral-encoded glycoprotein, gp64 Keddie, B. A., Aponte, G. W., and Volkman, L. E. 1989. The pathway of infection of Autographa californica nuclear polyhedrosis virus in an insect host. Science 243, 1728-1730!. This budded virus (BV) spreads infection to the other tissues of the insect which produce both BV aid polyhedra. Insects die after virtually all their tissues are infected massively, and cadavers liquefy releasing numerous polyhedra which horizontally transmit the virus to susceptible larvae.
Although many species of lepidopteran larvae can support AcMNPV infections, there is an enormous variation among species with regard to mortality due to infection. Among species, the time between initial infection and virus induced death ranges from a few days to several weeks. In addition, results from laboratory studies have revealed that for susceptible hosts (e.g. Trichoplusia ni, Heliothis virescens, and Spodoptera exidua) a dozen or fewer polyhedra per insect may be sufficient to kill 50% or more of the larval population. In contrast, to achieve comparable mortality levels within populations of resistant hosts (e.g., Helicoverpa zea) each insect must ingest literally millions of polyhedra Vail, P. V. and S. S. Collier. 1982. Comparative replication, mortality, and inclusion body production of the Autographa californica Nuclear Polyhedrosis Virus in Heliothis sp. Ann. Entomol. Soc. Am. 75:376-382; Vail, P. V. and Vail, S. V. 1987. Comparative replication of Autographa californica nuclear polyhedrosis virus in tissues of Heliothis spp. (Lepidoptera: Noctuidae). Ann. Entomol. Soc. Amer. 80:734-738!.
Recent research efforts aimed at improving AcMNPV as a commercial microbial pesticide have focused primarily on reducing the time to death of infected hosts. The principal strategy employed for these efforts has been to genetically engineer AcMNPV recombinants that incorporate genes coding for invertebrate toxins and compounds that interfere with larval hormone function Bonning, B. C. and Hammock, B. D. 1992. Development and potential of genetically engineered viral insecticides. Biotechnology and Genetic Engineering Reviews 10:455-489; Maeda, S. 1994. Expression of foreign genes in insect cells using baculovirus vectors. pp 1-31. In (K. Maramorosch and A. H. McIntosh, eds.), Insect Cell Biotechnology, CRC Press, Boca Raton, Fla.!. Recombinants expressing these substances have been shown to reduce larval feeding damage, and the first field trials of such recombinants recently have been approved by EPA. By comparison, little progress has been made toward the goal of expanding the host range of AcMNPV, in part because little is known about the mechanisms of resistance. Indeed, there is apparently no published description or explanation of the physiological basis for variation in susceptibility to fatal AcMNPV infections.
Thus, there remains a need for compositions and methods which enable effective control (e.g., reduction or eradication) of a wider variety of insect populations.
It is an object of the present invention to provide compositions and methods which do not suffer from the drawbacks of the heretofore-known compositions.